The present invention relates to the field of optical fiber communications and, more particularly, to systems and methods for measuring/resolving polarization mode dispersion (PMD) in optical fibers.
Fiber dispersion (both chromatic dispersion and polarization mode dispersion) is an important measure in the design of optical fiber systems. In its most general terms, dispersion is defined as the separation of a beam into its various components.
In an optical fiber, dispersion occurs because the differing wavelengths propagate at differing speeds resulting in what is termed chromatic dispersion. PMD is more complex in that, in a real fiber, residual birefringence from the manufacturing process and that induced by bending and thermal effects create differing optical axis that generally correspond to the maximum and minimum of the refractive index profiles across the diameter of the fiber. Such axes are usually orthogonal due to the mechanical stress distribution and have different propagation constants. These axes can be thought of as corresponding to the linearly polarized (LP) polarization modes or principal states of polarization. Plane polarized light propagating along the fiber will be resolved into components in these axes and as they propagate at different speeds, phase differences are created resulting in elliptically polarized light. The sum of all phase change mechanisms along a fiber is the measure of polarization mode dispersion.
U.S. Pat. No. 5,956,131 titled System and Method for Mapping Chromatic Dispersion in Optical Fibers issued Sep. 21, 1999 to Mamyshev et al. is directed to measuring chromatic dispersion only. Manyshev et al. proposed launching first and second optical pulses repetitively into a fiber under test to generate, by a four-wave mixing (FWM) process in the fiber, a probe signal. Because of the wave-vector phase mismatch, the probe signal power oscillates with a spatial frequency that can be measured as a function of distance in the fiber. These intensity oscillations are measurable as, for example, temporal variations in the Rayleigh backscattered light detected at the input end of the fiber. The dispersion parameter at one or both of the first and second optical signal wavelengths, as a function of length along the fiber, is derived directed from these intensity oscillations measurements.
In summary, Manyshev et al. calculates chromatic dispersion from a detected back-reflected Rayleigh signal at either the Stokes or anti-Stokes frequency, which has the wave-vector phase mismatch information. The wave-vector phase mismatch results from the fiber dispersion at one of the two source wavelengths. The wave-vector phase mismatch equals zero when the dispersion is zero. The polarization of the two laser sources launched into the fiber under test has to be aligned (i.e. co-polarized) to maximize the four-wave mixing products.
In accordance with one aspect of the present invention there is provided a method of resolving polarization mode dispersion in a span of optical fiber, comprising the steps of: (a) launching into a first end of the fiber, simultaneously, a first optical signal at a first wavelength and a second optical signal at a second wavelength such that the first and second wavelength are co-polarized at a first state of polarization to generate a first probe signal using a four wave mixing process in which two photons at the first wavelength combine with one photon at the second wavelength; (b) measuring the frequency of oscillations of the first probe signal as a function of distance in the fiber to generated a first measured dispersion map; (c) launching into the first end of the fiber, simultaneously, the first optical signal at the first wavelength and the second optical signal at the second wavelength such that the first and second wavelength are co-polarized at a second state of polarization to generate a second probe signal using a four wave mixing process in which two photons at the first wavelength combine with one photon at the second wavelength; (d) measuring the frequency of oscillations of the second probe signal as a function of distance in the fiber to generate a second measured dispersion map; and (e) deriving from the first and second measuring dispersion maps group velocities as a function of wavelength; and (f) calculating the difference between the group velocities of step (e) to obtain the polarization mode dispersion in the span of optical fiber.
In accordance with another aspect of the present invention there is provided a method of resolving polarization mode dispersion in a span of optical fiber, comprising the steps of: (a) launching into a first end of the fiber, simultaneously, a first optical signal at a first wavelength and a second optical signal at a second wavelength such that the first and second wavelength are co-polarized to generate a first probe signal using a four wave mixing process in which two photons at the first wavelength combine with one photon at the second wavelength; (b) measuring the frequency of oscillations of the first probe signal as a function of distance in the fiber; (c) deriving, for at least one of the first and second wavelengths, a chromatic dispersion parameter as a function of distance along the fiber from the measurement obtained from step (b); (d) launching into the first end of the fiber, simultaneously, the first optical signal at the first wavelength and the second optical signal at the second wavelength such that the first and second wavelength are at different states of polarization to generate a second probe signal using a four wave mixing process in which two photons at the first wavelength combine with one photon at the second wavelength; (e) measuring the frequency of oscillations of the second probe signal as a function of distance in the fiber; (f) repeating steps (d) and (e) at a plurality of different state of polarizations; (g) deriving, for at least one of the first and second wavelengths, a dispersion parameter representing a combination of the chromatic dispersion parameter and polarization mode dispersion as a function of distance along the fiber from the measurements obtained from steps (e) and (f); and (h) deriving from the dispersion parameter obtained at step (g) and from the chromatic dispersion parameter obtained at step (c) the polarization mode dispersion in the span of optical fiber.
In accordance with another aspect of the present invention there is provided an apparatus for resolving polarization mode dispersion in a span of optical fiber, comprising: (a) an optical signal generating arrangement for launching into a first end of the fiber, simultaneously, a first optical signal at a first wavelength and a second optical signal at a second wavelength at a first and a second co-polarized state of polarization to generate probe signals via a four wave mixing process where for each probe signal two photons at the first wavelength combine with one photon at the second wavelength; (b) a detecting arrangement for measuring the frequency of oscillations of the probe signals as a function of distance in the fiber, where for at least one of the first and second wavelengths at each of the first and second co-polarized states of polarization, dispersion maps are derived as a function of distance along the fiber; and (c) a calculating arrangement for deriving from the dispersion maps group velocities as a function of wavelength with polarization mode dispersion being the difference between the group velocities.
In accordance with another aspect of the present invention there is provided an apparatus for resolving polarization mode dispersion in a span of optical fiber, comprising: (a) an optical signal generating arrangement for launching into a first end of the fiber, simultaneously, a first optical signal at a first wavelength and a second optical signal at a second wavelength at (i) a co-polarized state of polarization and at (ii) a plurality of different states of polarization to generate probe signals via a four wave mixing process where for each probe signal two photons at the first wavelength combine with one photon at the second wavelength; and (b) a detecting arrangement for measuring the frequency of oscillations of the probe signals as a function of distance in the fiber, where for at least one of the first and second wavelengths, (i) a chromatic dispersion parameter is derived as a function of distance along the fiber from (a)(i) and (ii) a dispersion parameter is derived as a function of distance along the fiber from (a)(ii) such that polarization mode dispersion being the difference between the dispersion parameter and the chromatic dispersion parameter.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.